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Universal Shipborne Automatic Identification System (AIS) Transponder

What is an AIS?

Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading).  Each ship "mark" could reflect the actual size of the ship, with position to GPS or differential GPS accuracy.  By "clicking" on a ship mark, you could learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information.  Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available.  Display information previously available only to modern vessel traffic service operations centers could now be available to every AIS-equipped ship.  

With this information, you could call any ship over VHF radiotelephone by name, rather than by "ship off my port bow" or some other imprecise means.   Or you could dial it up directly using GMDSS equipment.  Or you could send to the ship, or receive from it, short safety-related email messages.

The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds.  It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.  

How Does it Work?

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link (IEC 61162/NMEA 0183) to shipboard display and sensor systems. Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters.  Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections.  Heading information and course and speed over ground would normally be provided by all AIS-equipped ships.  Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided.

The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas.  Transmissions use 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels using HDLC packet protocols. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.

    Each station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations.   A position report from one AIS station fits into one of 2250 time slots established every 60 seconds.  AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions.   Slot selection by an AIS station is randomized within a defined interval, and tagged with a random timeout of between 0 and 8 frames.  When a station changes its slot assignment, it pre-announces both the new location and the timeout for that location.  In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels. 

AIS Slots

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.

The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles. With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.

The system is backwards compatible with digital selective calling systems, allowing shore-based GMDSS systems to inexpensively establish AIS operating channels and identify and track AIS-equipped vessels, and is intended to fully replace existing DSC-based transponder systems.

What is broadcast by AIS

A Class A AIS unit broadcasts the following information every 2 to 10 seconds while underway, and every 3 minutes while at anchor at a power level of 12.5 watts. The information broadcast includes:

In addition, the Class A AIS unit broadcasts the following information every 6 minutes: 

Types of AIS

ITU-R Recommendation M.1371-1 describes the following types of AIS:

Class A

Shipborne mobile equipment intended for vessels meeting the requirements of IMO AIS carriage requirement, and is described above.

Class B

Shipborne mobile equipment provides facilities not necessarily in full accord with IMO AIS carriage requirements.  IEC has begun work on a Class B certification standard, which should be completed by 2003.  The Class B is nearly identical to the Class A, except the Class B:

Search and Rescue Aircraft

Aircraft mobile equipment, normally reporting every ten seconds.

Aids to Navigation

Shore-based station providing location of an aid to navigation.  Normally reports every three minutes.  This may eventually replace the racon.

AIS base station.  

Shore-based station providing text messages, time synchronization, meteorological or hydrological information, navigation information, or position of other vessels.  Normally reports every ten seconds.

AIS Frequencies

The International Telecommunications Union World Radio Conference in 1997 designated two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B) for AIS.   In the US, the first channel is owned by MariTEL, a public coast station operator, and the second by the federal government.  The USCG signed a Memorandum of Agreement with MariTEL for use of AIS 1, and has authority from the National Telecommunications and Information Administration to use both AIS1 and AIS 2 US-wide for AIS operation.  The USCG has asked the Federal Communications Commission to authorize any US vessel to operate AIS on these two channels under its existing ship station license.  The FCC released a Notice authorizing operation of AIS under a ship's existing station license.

IMO Carriage Requirement

The 73rd Session of the IMO's Maritime Safety Committee decided the following ships will be required to carry AIS equipment:

All ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with AIS, as follows:

The United States Coast Guard will also require AIS on certain vessels not subject to SOLAS under a forthcoming rulemaking.

What Standards Exist for AIS?

This standard defines the basic performance requirements for AIS equipment, and was used by ITU and IEC in developing technical and test standards.  It can be downloaded at no charge as an annex to the MSC 69 final report.

The International Telecommunications Union Sector for Radiocommunications formally adopted this standard in August 2001.  This is the standard that defines in detail how the AIS works, and as such is the primary AIS standard.  It can be purchased electronically from ITU.  ITU gave IALA the responsibility of maintaining technical guidelines for AIS design, and is available from them as "IALA Technical Clarifications on Recommendation ITU-R M.1371-1".

The International Electrotechnical Commission formally adopted this standard in November 2001, and a published version is available from IEC.  This standard defines the certification test requirements for Class A shipborne AIS equipment.

This 159-page document is available from IALA.

Certification of AIS equipment in the U.S.

In June 2002, the FCC released a Notice entitled "Applications For Equipment Authorization Of Universal Shipborne Automatic Identification Systems To be Coordinated with U.S. Coast Guard To Ensure Homeland Security". Pending completion of FCC rulemaking, the FCC Laboratory will coordinate review of applications for certification of AIS equipment with the United States Coast Guard to ensure that the equipment meets all applicable international standards and requirements.  Essentially, AIS manufacturers must the requirements of the FCC's regulations for equipment authorization, 47 CFR 2 Subpart J (beginning 2.901), and the Coast Guard's Navigational and Vessel Inspection Circular (NVIC) 8-01, Approval of Navigation Equipment for Ships.  NVIC 8-01 describes the certification process for AIS and other navigation equipment described under the newly adopted SOLAS V.  The Federal Communications Commission has requested comments on how its rules should be amended to accommodate AIS certification, in a further Notice of Proposed Rulemaking under Docket PR 92-257.  Until these FCC rules are finally adopted, the procedures described in the FCC Notice and the NVIC should apply.

For more AIS information:

USCG Office of VTS Reference Library

Ports and Waterways Safety System

St Lawrence Seaway System AIS Project

UK Maritime and Coast Guard Agency Annex 17 (includes AIS Guidelines)

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